The long-term goal of this application is to investigate the role of maspin in prostate tumor progression. Maspin is a novel epithelial-specific serine protease inhibitor (serpin) with tumor suppressive activities. Our study in the last funding period yielded several major breakthroughs including: (i) the discovery of a specific cell surface-dependent interaction between maspin and the uPA/uPAR complex (ii) maspin sensitizes tumor, but not normal, cells to induced apoptosis; (iii) Maspin induces tumor redifferentiation in vivo; (iv) maspin inhibits prostate tumor growth in human bone microenviroment and inhibits prostate tumor-induced osteolysis and angiogenesis. Our additional evidence further suggests that maspin enhances cell-cell interaction, retards cellmatrix dissociation, and inhibits oxidative stress-induced reactive oxygen species (ROS) generation. While recent X-ray crystal structural analyses confirm that maspin has a general framework conserved among most serpins but is metastable, the evidence that systemic maspin knockout is lethal at embryogenesis clearly indicates a uniquely critical function of maspin that can not be complemented by other molecules. In view of the multifaceted functionalities of maspin, it is of paramount importance to identify its molecular partners. In our further studies, the multifunctional low density lipoprotein related protein (LRP), and pro-uPA were identified as potential extracellular targets of maspin, while glutathione S-transferase (GST), Heat shock protein-90 (Hsp90) and histone deacetylase 1 (HDAC1) were identified as potential intracellular targets of maspin. For the first time, all evidence seems to consistently support one mode of maspin action: it acts as a stress-responsive chaperone. Together with the novel biphasic maspin expression pattern in early prostate cancer development, our data led to a novel Hypothesis that maspin plays an important role in the regulation of epithelial response to changes of the microenvironment. Three aims are designed to test this hypothesis. Specific Aim 1: To test the hypothesis that maspin chaperones and regulates the LRP-controlled cell surface presentation and activity of external stimuli; Specific Aim 2: To investigate the molecular mechanisms underlying the maspin effects on GST, Hsp90 and HDAC1-mediated intracellular stress-response pathways; and Specific Aim 3: To investigate the biological effect of maspin in prostate development and tumor progression in vivo. The proposed study is expected to generate paradigm-shifting evidence regarding the molecular mechanism of maspin. Consequently, the expected results will further define the potential utility of maspin-based therapeutic strategies